Business ethicist: Managers of corporations have an obligation to serve ... ...

Business ethicist: Managers of corporations have an obligation to serve shareholders as the shareholders would want to be served. Therefore, corporate managers have an obligation to act in the shareholders’ best interest. The business ethicist’s conclusion follows logically if which one of the following is assumed?
(A) Corporate managers are always able to discern what is in the best interest of shareholders.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Oct 2012 LSAT (PT67): Logical Reasoning B